From the Q&A

Strobe Alley Needs Power

Question:

The lights will be wired in two circuits, with nine lights on each leg. The distance to the first light is 500 feet, after which each light will be 330 feet apart. I plan on running #4 AWG copper wire for 1,490 feet then switch to #6 AWG copper wire for the remainder of the run. Any help you can give me to find a supplier of this transformer would be appreciated

Robert Mitten
via Internet

Answer:

First of all, I wouldn't use a 24-volt distribution line for this application. Instead, I'd go with a 120 VAC line and tap the line at each strobe location using a small 24-volt, 1-amp transformer, like the Alltronics (408-943-9773; www.alltronics.com) 97N002 or Mouser (800-346-6873; www.mouser.com) 553-F46X.

This accomplishes three desirable goals. First and foremost, it reduces the size of the wire from 4 gauge to 14 gauge — typical house wire. Observe: The current requirement for the 24-volt system is about 8 amps per leg, and the resistance of 4-gauge wire is 0.267 ohms per 1,000 feet, which gives a voltage drop of 1.1 volts in the 500-foot section. (E = IR = 8*0.134 = 1.1 volts.) That's nearly 5 percent of the power lost in the wiring alone. The current of the 120-volt system, on the other hand, is just 2 amps per leg. With a resistance of 2.73 ohms per 1,000 feet, the voltage drop across 500 feet of 14-gauge wire is just 2.73 volts, or about 2 percent of the total power. Furthermore, you can use 14-gauge wire throughout the system, eliminating the need to mix two heavy-gauge wires. Last, but not least, 4-gauge wire costs 43 cents per foot, ($4,428.00 total with 1650 feet of #6 @ 28 cents per foot), whereas 14-gauge wire costs 5 cents per foot, ($630.00 total), for a savings of about $3,800.00. If a 120-volt system isn't practical because of building codes, the line voltage can be reduced to 72 or 48 volts, as shown in the wiring diagram above. Here are two charts showing wire resistance versus wire size, and recommended wire sizes for the three above systems.

Gauge (AWG)

Diameter (mm)

Area (cir mils)

Resistance (1000 ft.)

0

8.252

53.49

0.105

2

6.543

33.62

0.168

4

5.189

21.15

0.247

6

4.115

13.30

0.425

8

3.264

8.367

0.687

10

2.588

5.261

1.08

12

2.05

3.310

1.71

14

1.63

2.081

2.73

16

1.29

1.3088

4.35

18

1.02

0.8231

6.92

20

0.81

0.5176

10.9

22

0.64

0.3255

17.5

24

0.51

0.2047

27.7

26

0.40

0.1287

44.4

28

0.32

0.0810

70.7

30

0.25

0.0509

112

32

0.20

0.0320

176

34

0.16

0.0201

280

36

0.13

0.0217

445

38

0.10

0.0080

708

40

0.08

0.0050

1093

Recommended Wire Sizes

System Voltage

Total Amps

Amps Per Leg

Voltage Drop (first 500 ft.)

Wire Size

120

3.06

1.53

2.09/2%

14

72

4.09

2.05

1.75/2.4%

12

48

7.15

3.58

1.23/2.6%

8

24

15.3

7.15

0.96/4%

4

If you insist on using the 24-volt system you describe, the transformer you want is called a furnace or power control transformer. They are available from Allied Electronics and Newark Electronics, but are very costly. You can save money by cruising the surplus electronic outlets, like All Electronics (www.allelectronics.com) — subject to availability, of course. (Note: Any 24-volt transformer with a 16-amp secondary or greater will work.) A better bet is to use two 8-amp, 24-volt transformers, one for each line, rather than a 16-amp behemoth. They're easier to find and cheaper.